hezz, uCoz provide a maximum of 10000MB of space per each website after you have paid for the increase. It is not possible to increase the sites disk-space after you have reached this limit. 10000MB of disk space however is a massive amount of usage already, if you don't mind me asking for what reason exactly do you want the extra disk-space?
As to utilizing other hosting companies it is possible. Cross-system usage is not that common though and can cause unnecessary load on the servers especially if you are sending a lot of requests to different hosts.
Jack of all trades in development, design, strategy.
Working as a Product Support Manager.
Been here for 9 years and counting.